This invention relates to an electrically operated system that conditions air to be used in occupied building space so that it is both cooled and dehumidified. The optimum temperature range for human comfort is well established while the humidity range is recognized but not universally applied. For example, heretofore a range of 20% to 80% was thought to be permissive, while a more optimum range of 40% to 60% is now shown to be required in order to minimize and/or eliminate bacterial, viral and fungal growth. Humidity has its affect upon air cleanliness, as it reduces the presence of dust particles, and the deterioration of building structure and contents, otherwise subjected to excess moisture. Accordingly, humidity control becomes an important factor as related to both human comfort and health, and to structural longevity as well.
Electrically powered mechanical refrigeration is employed here for sensible cooling, and desiccant dehumidification is combined therewith. Supply air (SA) to the building space is from both outside air (OSA) and return air (RA), the proportion thereof being a greater amount of return air and a minimized amount of outside air, for example 75% RA to 25% OSA. The minimum outside air is dehumidified and mixed with return air and controlled by a damper prior to cooling by means of direct expansion in evaporator coils of the refrigeration system, the air cooled or chilled thereby being discharged into the building space as supply air (SA).
A feature of this invention is the desiccant dehumidifier that is regenerated by a column of heated air tempered by waste heat of the refrigeration system, a mechanical system with a condenser coil thereof in the heated air column at the regeneration side of the dehumidifier. It is to be understood that a solid desiccant or liquid desiccant dehumidifier can be employed as desired. In practice, a solid desiccant system is prefered, of the dynamic type and of rotary bed configuration, wherein the bed is a wheel comprised of a screen of tubes or plates of solid desiccant to which air can be continuously exposed, progressively exposed to a dehumidifying air duct and then to a regeneration air duct. The dehumidifying is from minumum outside air (OSA) to be mixed with return air (RA) and then cooled and discharged as supply air (SA). The regeneration air duct is also from outside air (OSA) to be discharged to ambient as exhaust (EXH). The greater portion, approximately 2/3 of the rotary bed of desiccant is exposed to the dehumidifying duct, while the lesser portion, approximately 1/3, is exposed to the regenerating air duct. The heated air regeneration duct is supplied with outside air (OSA) by means of a blower, and the air heated by a condenser coil, and then discharged to atmosphere (EXH) after regeneration of the desiccant. The air ducts are sealed with the rotary bed of desiccant which is rotated slowly by a variable speed motor.
By using a solid desiccant to precondition the minimum outside air (OSA) it is possible to operate the evaporator coil at a substantially elevated temperature as compared with the conventional mechanical refrigeration systems, thereby reducing net energy requirements for the same net cooling load. When air is dried with desiccants, it's temperature rises because the latent heat and heat from the regenerated desiccant and its carrier are transferred to the dried air. Therefore, it is an object of this invention to compensate for this heat rise effect by providing an after cooling means associated with the exhaust of excess return air (RA) to atmosphere, utilizing a cooling effect therefrom to remove heat leaving the dehumidifying desiccant. thereby reducing load on the downstream refrigerant evaporation coil. The relative cooling requirements of the different available desiccants relate directly to the amount of regenerating energy employed, because of the carryover of heat by the desiccant and associated structure. This heat, plus the heat of condensation, must be removed by cooling. As a result, relative cooling requirements for the different types of commercially available dehumidifiers represent a significant portion of the regeneration energy requirement.
In accordance with this invention, the after cooling means is embodied in several forms. The excess discharge of return air (RA) is at a relatively low temperature and of relatively low humidity, coming from the conditioned air space, and is subject to being evaporatively cooled. Accordingly, it is this exhaust air flow which is advantageously utilized to absorb heat from heated air leaving the desiccant dehumidifier; in one embodiment by means of evaporatively cooled air flowing through a heat exchanger; and in one embodiment by means of heat pipes extending between cold side and hot side heat exchangers in the return air exhaust duct and in the desiccant dehumidifier delivery duct. As will be described, the evaporative cooler and/or heat pipes transfer heat energy between the outgoing and incoming air.
Desuperheating is used to supply liquid desiccant in the liquid desiccant embodiment herein disclosed, to spray cooler desiccant liquid into the humidifying section thereof. The dehumidifying fluid circuit is associated with the refrigeration compressor as will be described.